Assembly of an aluminum component and of a steel component having a znalmg alloy coating

ABSTRACT

The present invention provides an assembly of an aluminum-based element and an element made of steel provided on at least one of the surfaces thereof with a metal coating. The metal coating is made of a zinc-aluminum-magnesium alloy including from 2.3% to 3.3% by weight of magnesium and from 3.5% to 3.9% by weight of aluminum. The remainder of the metal coating is zinc, inevitable impurities and possibly one or more additional elements selected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi. The coated surface of the steel element is in at least partial contact with the the aluminum-based element. The contact is brought about by a layer of adhesive or sealant having a thickness of less than 5 mm. The present invention also provides a part for a vehicle including at least one assembly and a vehicle.

The present invention relates to an assembly of an aluminum-based partand a part made of steel provided with a coating made of azinc-aluminum-magnesium alloy, on at least one of the surfaces thereofplaced so as to be in contact at least partially with the aluminum-basedpart.

These assemblies are for example intended for use in the manufacture ofauto body parts, such as door openings and the like, without howeverbeing limited thereto.

BACKGROUND

The metal coatings essentially comprising zinc are traditionally usedfor their good protective properties against corrosion. The addition ofmagnesium to the coating markedly increases the corrosion resistance ofthese coatings, which then provides the ability to reduce theirthickness or provide greater assurance of protection against corrosionover time. Furthermore, the addition of aluminum also makes it possibleto improve the corrosion resistance.

The constant need to seek reduction in weight in order to reduce CO2emissions leads to the need to seek hybrid solutions combining inparticular aluminum and zinc coated steel. However, such a combinationgenerates complex and evolving phenomena of corrosion by galvaniccoupling of the two materials.

The patent application EP 2 141 255 discloses assemblies wherein theprocess involves adding magnesium to the steel coating in a quantitywhich may in particular vary between 0.8% and 10% by weight. Theaddition of 0.8% to 3.5% of aluminum to this same coating may also beenvisaged.

BRIEF SUMMARY

However, the corrosion resistance of these assemblies continues toremain far too low over time, and there therefore exists a need forimprovement thereof.

The present invention provides an assembly of an aluminum-based elementand an element made of steel provided on at least one of the surfacesthereof, with a metal coating made of a zinc-aluminum-magnesium alloyincluding from 2.3% to 3.3% by weight of magnesium, from 3.5% to 3.9% byweight of aluminum, the remainder of the metal coating being zinc,inevitable impurities and possibly one or more additional elementsselected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi.The coated surface of the element made of steel is in at least partialcontact with the aluminium-based element, the contact being broughtabout by a layer of adhesive or sealant whose thickness is less than 5mm.

The assembly may also include the following features, alone or incombination:

the metal coating made of a zinc-aluminium-magnesium alloy includes from2.3% to 3.3% by weight of magnesium, and from 3.6% to 3.9% by weight ofaluminium, the remainder of the metal coating consisting of zinc,inevitable impurities and possibly one or more additional elementsselected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi;and

the aluminium-based element and the element made of steel are assembledby crimping.

The present invention also provides a part for a vehicle, a vehicle doorand a vehicle. The part includes the assembly according to the presentinvention. The vehicle door includes the part, the aluminium-basedelement is the exterior part of the door and the element made of steelis the interior part of the said door. The vehicle includes the part orthe vehicle door.

The present invention also provides use of a part made of steel providedon at least one of the surfaces thereof, with a metal coating made of azinc-aluminium-magnesium alloy including from 2.3% to 3.3% by weight ofmagnesium, from 3.5% to 3.9% by weight of aluminium, the remainder ofthe metal coating being zinc, inevitable impurities and possibly one ormore additional elements selected from among Si, Sb, Pb, Ti, Ca, Mn, Sn,La, Ce, Cr, Ni or Bi, for the manufacture of assemblies in accordancewith the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be illustrated by means of indicative examplesgiven for information purposes only, and without limitation, withreference made to the accompanying figures in which:

FIG. 1 schematically represents an assembly according to the invention;

FIG. 2 represents the measurement curves depicting the evolution of theaverage depth of corrosion of the aluminum panels within the assembliesaccording to the invention in comparison to assemblies according to theprior art,

FIG. 3 represents the measurement curves depicting the evolution of thecurrent density of galvanic coupling of assemblies according to theinvention in comparison to assemblies based on the prior art.

DETAILED DESCRIPTION

The assembly 1 according to an embodiment of the present invention firstof all comprises a first aluminum-based element, and which here takesthe form of a panel 2. By that it is intended to cover pure aluminum andall of its alloys comprising at least 85% by weight of aluminum,including therein the 1000 to 7000 series which comprise aluminumwithout any alloying additive element and alloys of variouscompositions:

aluminum without alloying element: 1000

aluminum+copper: 2000

aluminum+manganese: 3000

aluminum+silicon: 4000

aluminum+magnesium: 5000

aluminum+magnesium+silicon: 6000

aluminum+zinc+magnesium: 7000

This panel 2 has suitable dimensions adapted to the subsequent use ofthe assembly according to the invention. In a preferred embodiment, theassembly 1 in question is integrated into a vehicle door, thealuminum-based panel being positioned on the exterior side of thevehicle.

The aluminum-based panel 2 may further include on at least one part ofits surface one or more protective coatings such as phosphating and/orcataphoresis type coatings, such as those usually applied on thebodies-in-white for vehicles. The assembly with the second steel coatedelement 3 being generally performed prior to the passage of thebody-in-white in the baths containing coating types like phosphatingand/or cataphoresis coatings, only the parts situated outside theassembly zone will thus be coated.

The second element of the assemblies according to the invention is thusan element 3 made of steel provided on at least one of the surfacesthereof, with a metal coating made of a zinc-aluminum-magnesium alloyincluding from 2.3% to 3.3% by weight of magnesium, from 3.5% to 3.9% byweight of aluminum, the remainder of the metal coating consisting ofzinc, inevitable impurities and possibly one or more additional elementsselected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi.

The content by weight of each additional element is generally less than0.3%. The additional elements may, among other things, provide theability to improve the ductility or adhesion of the coating on to thesteel element. The person skilled in the art who knows their effects onthe characteristics of the coating will know to use them according tothe additional or complementary desired goal.

The coating may finally contain residual elements originating, forexample, from the coating bath when use is made of a hot dip coatingprocess. It may be contaminated with impurities derived from the feedingots or resulting from the passage of the steel element in the coatingbath. Among these mention may be made in particular of iron the contentwhereof may go up to 5% by weight and is generally between 2% and 4% byweight in the coating.

In a preferred embodiment, the coating comprises from 2.3% to 3.3% byweight of magnesium and from 3.6% to 3.9% by weight of aluminum.

In a preferred embodiment, the weight ratio between the magnesium andaluminum in the metal coating is strictly less than or equal to 1,preferably strictly less than 1, and more preferably strictly less than0.9.

The metal coating generally has a thickness that is less than or equalto 30 (microns) or even 25 μm and greater than or equal to 3 μm or even5 μm.

As previously mentioned above, the two elements 2, 3 are assembled in amanner such that the coated surface of the element 3 made of steel is inat least partial contact with the aluminum-based element 2. In apreferred embodiment, the assembly may be done in particular by amechanical method such as crimping of the two elements in an assemblyzone 5 located, in the example shown in FIG. 1, in the lower part of thedoor formed by the assembly 1.

Such crimping could consist of a simple folding of one of the elementsaround the other, as shown in FIG. 1 where the element 2 is arranged soas to envelope the element 3. It is of course possible to carry out anyother type of mechanical assembly known to the person skilled in theart.

It is also possible to assemble the panels according to the invention bymeans of structural bonding making use of an adhesive or a structuralsealant which enables an assembly such that the mechanical stresses towhich one or the other of the elements 2, 3 are subjected gettransferred to the other element 3, 2. The layers of adhesives and/orsealants being very thin (generally less than 5 mm, or even less than 1mm and even less than 200 μm), it would be considered that such a bondedassembly amounts to putting in contact the two elements 2, 3 in the sameway as a simple mechanical assembly.

It is quite obviously possible to combine structural bonding andmechanical assembly in order to enhance the robustness of the assembly.

With a view to highlighting the enhanced performance obtained throughusing the assemblies according to the invention, some actual examples ofembodiments will be detailed in comparison with assemblies based on theprior art.

EXAMPLES

For all of the examples of embodiments detailed, assembly samples werecreated with the assembly of panels made of aluminum AA6061 and steelpanels coated with aluminum-zinc-(magnesium) alloys of variablecomposition, the coatings having a uniform thickness of 7.5 μm.

These assemblies include a first coated steel panel, having a size of 10cm in width by 20 cm in height, for example. The process involvesassembly by bonding of an aluminum panel having a size of, for example,10 cm in width by 4 cm in height and having a thickness of 1 mm. Thealuminum panel is positioned at the center of the steel panel and thespacing between the two panels due to the presence of adhesive is, forexample, 120 which simulates an assembly such as that of a motor vehicledoor. The active zone where the two unpainted sidings face each otherhas a dimension of, for example, 6 cm in width by 4 cm in height. Acopper wire connects the two sidings in order to bring them intoelectrical contact and thus be able to simulate galvanic corrosion inthe active zone.

The inactive zone of the two sidings is protected by a paint obtained byphosphating followed by cataphoresis. The phosphating is performed inthree steps with the products from the company Chemetall, for example,:(i) degreasing agent Gardoclean S 5176+additive H7352, (ii) refinerGardolene ZL6, (iii) phosphating bath Gardobond R24TA. The cataphoresiswas performed with products from the company PPG, for example,: bindingagent W772+additive P978; T=32° C.; t=150 s ; U=200V; Cooking: T=180°C.; duration time =35 min; cataphoresis thickness: 20

Corrosion Test

The test used is the VDA 233-102 test which provides for subjecting thesamples to a series of treatment cycles, each cycle lasting for oneweek, with a succession of sub-cycles A, B and C of 24 hours each.

-   -   Cycle A: cycle with treatment for a period of three hours by        placing in the presence of a salt spray mist at 35° C.;    -   Cycle B: cycle of 24 hours without treatment with a salt spray        mist at a temperature varying between 25° C. and 50° C. and a        relative humidity varying between 70% and 95%;    -   Cycle C: cycle without treatment with a salt spray mist, at a        temperature varying between −15° C. and 50° C. and a relative        humidity varying between 70% and 95% ring

The saline solution used is an aqueous solution containing 1% by weightof sodium chloride.

Galvanic Coupling Test

In order to evaluate the galvanic current density generated by theassembly of the aluminum and the coated steel, it is necessary toelectrically connect the aluminum panel in the zone coated bycataphoresis treatment and the lower zone of the coated steel panel, byinterposing a zero resistance ammeter (Bio-Logic Potentiostat VMP3).

Laser Triangulation Test

In order to evaluate the degree of corrosion of the zones of the coatedsteel panel included amongst the assembly samples described previously,measurements are carried out to determine the depth of corrosion etchingof the aluminum by laser triangulation over the entire surface of asample, and then the maximum observed value in micrometers is extractedtherefrom.

Tests

The assembly samples of aluminum panels and steel panels coated withzinc-aluminum-(magnesium) alloys of variable composition such aspreviously described above were made and then subjected to the VDA233-102 test.

At the end of each cycle, a first sample was subjected to measurementsof depth of corrosion etching of the aluminum element by a lasertriangulation technique.

The results at 6 and 12 cycles are summarized and presented in Table 1and the results detailed, cycle by cycle are presented in FIG. 2.

A second sample was subsequently subjected to measurement of galvaniccoupling potential. The results at 6 and 12 cycles are summarized andpresented in Table 2 and the results detailed, cycle by cycle arepresented in FIG. 3.

TABLE 1 Maximum depth of corrosion etching % Al by % Mg by P (μm) P (μm)Sample weight weight after 6 cycles after 12 cycles 1 0.3 0 550 660 21.5 1.5 460 570 3 3.7 3.0 30 310

It is found that the sample 3 makes it possible to obtain significantlyfar better results than the Comparative Tests 1 and 2. Certainly someimprovement in corrosion resistance is indeed observed between thesample 2 and the sample 1 after 12 cycles, however the improvementobtained in the sample 3 is of a significantly greater magnitude andaltogether quite surprising. In addition, if one were to consider theresults upon the completion of 6 cycles, there is clear indication of adifference in behavior that is extremely significant between the testaccording to the invention and the comparative tests, the maximum depthof corrosion etching of the aluminum element being divided by a factorof more than 15.

TABLE 2 Galvanic Coupling % Al by % Mg by Q (μA/cm²/h) Q (μA/cm²/h)Sample weight weight after 6 cycles after 12 cycles 1 0.3 0 3 800   8470 2 1.5 1.5 540 5 900 3 2.0 2.0 490 5 900 4 3.7 3.0  80   330

It is noted that the sample 4 makes it possible to obtain significantlyfar better results than the Comparative Tests 1 to 3. Certainly someimprovement in corrosion resistance is indeed observed between thesamples 2 and 3 and the sample 1, however the improvement obtained inthe sample 4 is once again in this case, of a significantly greatermagnitude and altogether quite surprising, especially since the sample3, containing higher aluminum and magnesium content than sample 2,presents in the final analysis, performance levels that are identical tothose of sample 2, which does not lead one to expect that an additionalincrease in aluminum and magnesium content would improve the performancelevels, especially in these proportions.

1-7. (canceled)
 8. An assembly comprising: an aluminum-based element;and a steel element provided on at least one of the surfaces thereofhaving a metal coating made of a zinc-aluminum-magnesium alloy includingfrom 2.3% to 3.3% by weight of magnesium, from 3.5% to 3.9% by weight ofaluminum, a remainder of the metal coating being zinc and inevitableimpurities; the coated surface of the steel element is in at leastpartial contact with the aluminum-based element via a layer of adhesiveor sealant having a thickness of less than 5 mm.
 9. The assemblyaccording to claim 8, wherein the metal coating further includes one ormore additional elements selected from among Si, Sb, Pb, Ti, Ca, Mn, Sn,La, Ce, Cr, Ni or Bi.
 10. An assembly according to claim 8, wherein themetal coating made of a zinc-aluminum-magnesium alloy includes from 2.3%to 3.3% by weight of magnesium, and from 3.6% to 3.9% by weight ofaluminum, the remainder of the metal coating being zinc and inevitableimpurities.
 11. The assembly according to claim 10, wherein the metalcoating further includes one or more additional elements selected fromamong Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi.
 12. The assemblyaccording to claim 8, wherein the aluminum-based element and the steelelement are assembled by crimping.
 13. A part for a vehicle comprisingat least one assembly according to claim
 8. 14. A vehicle doorcomprising: the part according to claim 13, the aluminum-based elementforming an exterior part of the door, and the steel element forming aninterior part of the door.
 15. A vehicle including at least one partaccording to claim
 13. 16. Use of a part made of steel provided on atleast one of the surfaces thereof, with a metal coating made of azinc-aluminum-magnesium alloy including from 2.3% to 3.3% by weight ofmagnesium, from 3.5% to 3.9% by weight of aluminum, the remainder of themetal coating being zinc and inevitable impurities, for the manufactureof assemblies according to claim
 8. 17. The use of a part according toclaim 16 wherein the metal coating further includes one or moreadditional elements selected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La,Ce, Cr, Ni or Bi.
 18. The assembly according to claim 10, wherein thealuminum-based element and the steel element are assembled by crimping.19. A vehicle including at least one vehicle door according to claim 14.20. Use of a part made of steel provided on at least one of the surfacesthereof, with a metal coating made of a zinc-aluminum-magnesium alloyincluding from 2.3% to 3.3% by weight of magnesium, from 3.5% to 3.9% byweight of aluminum, a remainder of the metal coating consisting of zinc,inevitable impurities and possibly one or more additional elementsselected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi,for the manufacture of assemblies according to claim
 10. 21. Use of apart made of steel provided on at least one of the surfaces thereof,with a metal coating made of a zinc-aluminum-magnesium alloy includingfrom 2.3% to 3.3% by weight of magnesium, from 3.5% to 3.9% by weight ofaluminum, the remainder of the metal coating consisting of zinc,inevitable impurities and possibly one or more additional elementsselected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi,for the manufacture of assemblies according to claim
 12. 22. Use of apart made of steel provided on at least one of the surfaces thereof,with a metal coating made of a zinc-aluminum-magnesium alloy includingfrom 2.3% to 3.3% by weight of magnesium, from 3.5% to 3.9% by weight ofaluminum, the remainder of the metal coating consisting of zinc,inevitable impurities and possibly one or more additional elementsselected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi,for the manufacture of parts according to claim
 13. 23. Use of a partmade of steel provided on at least one of the surfaces thereof, with ametal coating made of a zinc-aluminum-magnesium alloy including from2.3% to 3.3% by weight of magnesium, from 3.5% to 3.9% by weight ofaluminum, the remainder of the metal coating consisting of zinc,inevitable impurities and possibly one or more additional elementsselected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi,for the manufacture of vehicle doors according to claim
 14. 24. A methodof manufacturing the assembly recited in claim 8 comprising the stepsof: supplying the aluminum-based element; and supplying the steelelement provided on at least one of the surfaces thereof having a metalcoating made of a zinc-aluminum-magnesium alloy including from 2.3% to3.3% by weight of magnesium, from 3.5% to 3.9% by weight of aluminum, aremainder of the metal coating being zinc and inevitable impurities;contacting, at least partially, the aluminum-based element with thecoated surface of the steel element via a layer of adhesive or sealanthaving a thickness of less than 5 mm.
 25. The method of manufacturingaccording to claim 25, wherein the metal coating further includes one ormore additional elements selected from among Si, Sb, Pb, Ti, Ca, Mn, Sn,La, Ce, Cr, Ni or Bi.